vm-allocator: Enable build with kvm feature

Currently `vm-allocator` module cannot be solely built, by add
`features` section and specifying `arch/kvm` to turn on the features
required for its dependency - `arch` module to build. Thus enabling
`vm-allocator` crate to be built and tested with command:

```sh
cargo build -p vm-allocator --features kvm
```

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

vm-allocator/Cargo.toml

@@ -4,6 +4,10 @@ edition = "2021"
 name = "vm-allocator"
 version = "0.1.0"
 
+[features]
+default = []
+kvm = ["arch/kvm"]
+
 [dependencies]
 libc = "0.2.167"
 vm-memory = { workspace = true }